ABSTRACT
Phenols containing hydrogen-bonded OH groups are not sufficiently acidic for th is reaction to take place.
8.1.2 Colour form ation on reaction w ith alcoholic FeCl3 has been a very useful diagnostic test for phenols, and the colour obtained can give valuable clues w ith regard to the disposition o f the OH group(s). Ferric chloride is an oxidising as well as chelating agent and the colours form ed may be due to either of these factors or a com bination of the two. Thus, catechols which can undergo ready oxidation to o-quinonoid systems, give a green colour w ith FeCl3.2-Hydroxyacetophenone, salicylaldehyde and similar compounds possessing hydrogen-bonded OH groups give purple colour w ith FeCl3. Interestingly, highly acidic phenolic OH groups, as in 4-hydroxybenzoic acid, 4-hydroxyacetophenone and related com pounds give only a brown colour w ith FeCl3. M any students m ay not be aware of the fact that certain non-phenolic com pounds also give dark colours w ith FeCl3. For example, N, N -dim ethylaniline gives a blue colour due to oxidation.
